The invention relates generally to a method of fabricating a photomask and, more particularly, to a method of fabricating a halftone phase shift mask.
A semiconductor device is realized through various patterns on a wafer, and these patterns are generally formed through a photolithography process. According to a conventional photolithography process, a photoresist layer is formed on a layer to be patterned on a wafer and a predetermined portion is exposed using a photomask. Next, a photoresist layer pattern that exposes some of a surface of the layer to be patterned is formed by removing a portion of the photoresist layer, the solubility of which is changed or unchanged by the exposure, with a development process using a developing solution. Then, a pattern is formed by removing the exposed portion of the layer to be patterned with an etch process using the photoresist layer pattern as an etch mask. In this procedure, the pattern formed on the wafer is defined by the photoresist layer pattern and the photoresist layer pattern is formed as the pattern on the photomask is transferred thereto. Therefore, the pattern on the photomask layer is consequently transferred to the pattern on the wafer.
There are various kinds of photomasks, but a binary mask and a phase shift mask are widely used. The binary mask is easy to fabricate but has a limitation that it is difficult to transfer an image thereof onto a wafer when forming fine patterns. On the other hand, the phase shift mask causes a phase difference between light transmitted through a phase shift layer and light transmitted through a portion having no phase shift layer and generates destructive interference at an interface of the patterns by this phase difference, thereby representing relatively high resolution. Therefore, the phase shift mask is much more widely used as compared to the binary mask.
However, as the degree of integration of semiconductor devices increases, the design rule of the phase shift mask has also been reduced, and accordingly, the space between the patterns of the phase shift mask, i.e. the pattern pitch, has also been reduced. Due to this tendency, it becomes gradually more difficult to form the required fine pattern pitch of the phase shift mask. For example, while performing an electron beam exposure process, a development process, and an etch process, errors resulting from inaccuracies in patterns defined by an electron beam exposure apparatus, reactivity between a developing solution and a resist layer, or inaccuracies in the etch process are amplified, and the “Mean To Target” (MTT) consequently increases. Therefore, has become difficult to obtain a fine pattern pitch due to the gradually increasing MTT.